Superlattices from mismatched materials

Leo Esaki and Raphael Tsu at IBM pointed out in 1970 that imposing an artificial periodicity one or two orders of magnitude larger than the natural lattice spacing on a semiconductor crystal ought to yield novel and potentially useful electrical and optical effects. Since then, considerable work has been done on fabricating and investigating the properties of superlattices—epitaxially grown stacks of alternating thin layers (on the order of a hundred angstroms) of two different semiconductor materials. But until recently the requirement of good, defect‐free crystal matching at the interfaces has severely restricted the choice of materials used to grow such artificially periodic structures. Almost all superlattices were grown with alternating layers of GaAs and $Alx$ $Ga1−x$ As with the lattice constants (spacings) of the two materials differing by only about a part in a thousand.